The present invention relates in general to communication systems, and is particularly directed to a mechanism for measuring attenuation of a local telecommunications loop (twisted two-wire pair) serving customer premises equipment, and reporting the measured cable plant attenuation at a remote piece of digital data services (DDS) telephone equipment terminating the loop.
In the course of turn-up of network access services, it is often necessary to perform one or more test and measurement operations on the local loop (twisted wire pair), such as, but not limited to loop loss (attenuation), the presence of bridge-taps or load coils, and data integrity at different segments of the cable plant. Loop loss is customarily measured by placing a signal transmitter at one termination of the loop and a measurement device at the other (far) end of the loop. The signal generated by the transmitter, which may comprise a tone of known frequency 8, is received by the measurement device to determine the insertion loss across the bandwidth of interest for the service being deployed. The measured loop loss may then be compared with existing cable records or deployment guidelines for the network access equipment.
An obvious drawback to this measurement procedure is the need to employ two pieces of test equipment at the opposite ends of the loop, which may be separated by miles of communication cable. Also, some test equipment is capable of generating only a limited set of tones, which can limit testing capabilities for new services. For a particular service, the network""s access equipment may assist in troubleshooting the local loop, as many different types of equipment are capable of estimating loop loss of signal power, which may be reported through a control port.
In accordance with the present invention, there is provided a loop loss measurement and reporting mechanism that obviates the need to interactively access and/or conduct a test message exchange session with a test unit installed at a far end of the loop. Instead, the invention relies upon a priori knowledge of the operation and signal parameters of a signal-transmitting data port that terminates the far end of the loop, to conduct threshold detection and power level measurements.
At the digital data services (DDS) unit, the loop is terminated in a conventional manner through a transformer to a standard hybrid splitter having respective transmit and receive ports coupled to transmitter and receiver sections of the DDS unit. In order to monitor the loop, the hybrid""s receive port is coupled through a controlled gain analog signal amplifier to an analog-to-digital converter (ADC), which digitizes received analog signals for application to a downstream digital signal processor wherein code processing and analysis are conducted. The output of the ADC is coupled to the digital signal processor. through a controlled gain digital signal amplifier.
The gains of each of the amplifiers are controlled by the digital data service unit""s supervisory microcontroller. The outputs of the amplifiers are monitored by associated threshold detectors, while a power meter is coupled to monitor the power level in the digitized signal output of the digital signal amplifier. A front panel display, such as a light emitting diode (LED) array, which is typically employed for use with digital data services parameters is employed by the unit""s microcontroller during a loop loss test mode for displaying loop loss parameter information derived in the course of conducting power level measurements on the receive end of the cable plant.
To measure and report loop loss in the loop loss test mode, the digital data services unit turns off its transmitter for a prescribed period of time, after being powered up, or after a time-out associated with an initial training sequence from a data port at the far end of the loop. During this quiescent loop loss test mode period, the transmitter of the remote data port is active and transmits a continuous in-band signal of known spectral content and power. By in-band is meant a signal lying in a prescribed band of frequencies employed for DDS communications, so that a true measure of loop loss for the DDS operational parameters of the cable plant may be derived. Since the digital data services unit""s transmitter is turned-off, there is no potential problem from unpredictable echo power due to bridge-taps or wireline gauge changes. This receive-only condition allows the power level in the monitored signal to be measured and loop loss estimated.
In order to provide an estimate of loop loss through the cable plant, the receive path amplifier gains are initially set to prescribed xe2x80x98highxe2x80x99 gain values associated with a substantial amount of loop loss (e.g., a loop loss of at least 40 dB), and the threshold detector outputs are monitored. If neither detector threshold is exceeded, it is inferred that the cable plant suffers a substantial loop loss, and the monitored power level meter output is sampled and supplied to the test mode microcontroller. The sampled power level is compared with a set of values stored in a look-up table for the loop cable plant of interest, in order to derive an estimate of signal loss through the loop at the Nyquist frequency. The measured loop loss value is then used to selectively illuminate one or more LEDs of the display as a front panel indication of measured (loop loss) power level.
If either of the detector thresholds is exceeded for the first set of (maximum) amplifier gain settings, it is inferred that the loop loss is less than a maximum loss value, and the gain of the associated amplifier is iteratively decreased toward a lower gain value that permits a more accurate power estimate. After one or more adjustments of the gains of the analog and digital amplifiers to gain values that fall within a range associated with the actual loop loss, the measured power level is processed in accordance with associated look-up table parameters, to provide an estimate of loop loss, and the front panel""s LED display is controllably illuminated to display the measured power level.